The present invention relates to a method and a device for compensating for the accumulator pressure in an electrohydraulic braking system.
An electrohydraulic braking system is described, for example, in German Patent Application No. 195 48 248, which describes controlling the input of hydraulic fluid from a hydraulic pressure accumulator, via valves, into the particular wheel brake cylinders, the accumulator being charged by a pump.
German Patent No. 196 44 880 describes a method for operating a vehicle""s braking system, which includes a power brake unit, a pump for building up pressure in the wheel brakes, as well as a brake pedal. An operating force being applied to a brake pedal is recorded in a first step; the variable representing the braking action is recorded in a second step; it is determined in a third step, on the basis of the operating force and the variable representing the braking action, whether the braking action corresponds to the action expected by the brake pedal actuation, the braking action not corresponding to the expected action when the two variables deviate from one another; and a pressure buildup is triggered in a fourth step by actuation of the pump when the braking action does not correspond to the expected action.
In electrohydraulic braking systems (EHB), wheel pressures from a hydraulic high-pressure accumulator are applied via valves. Excessive strain on the brakes can lead to degradation of the braking performance (so-called brake fading). Given the same wheel circuit pressure, diminished braking action results in lower (or less effective) vehicle deceleration. When working with conventional braking systems, it is possible to compensate for this effect by applying an increased braking pedal force. In EHB systems, however, it can happen that the system""s hydraulic accumulator pressure does not suffice for applying this necessary wheel pressure.
Electrohydraulic braking systems are also discussed in the article entitled xe2x80x9cSAE Technical Paper Series 960991xe2x80x9d, International Congress and Exposition, Detroit, Mich., Feb. 26-29, 1996. This article discusses, inter alia, ascertaining a brake fading by observing a deceleration control. Provision can be made in such a case to raise the pressure level in the electrohydraulic braking system. It is believed that the method proposed in this article may be relatively expensive, since it may be necessary to compensate brake fading via special deceleration-control devices.
An object of an exemplary embodiment of the present invention is, therefore, to provide an electrohydraulic braking system, which will enable the required hydraulic accumulator pressure to be adjusted in a simple manner.
This object may be achieved by a method for compensating for the hydraulic accumulator pressure in an electrohydraulic braking system.
In accordance with an exemplary embodiment of the present invention, a method is provided for compensating for the accumulator pressure in an electrohydraulic braking system, in which a wheel setpoint pressure is compared in a simple manner to an active accumulator pressure, and in response to falling below (or failing to meet) a predefinable (or preset) minimum difference, the accumulator pressure is raised accordingly.
The variable representing the wheel setpoint pressure is expediently determined from a brake pedal travel, and/or a master brake cylinder pressure, and/or from the vehicle speed. These parameters are able to be determined relatively simply and make it possible for one to define an optimal wheel setpoint pressure with adequate precision.
It is also beneficial for a motor vehicle driver to be prompted when a compensation of accumulator pressure has taken place. It is believed that this measure raises the safety standard of the motor vehicle, since an occurring brake fading is able to be indicated, allowing the driver, if needed, to correct his driving technique or seek out a service station.